Numerical simulations have been performed to study the effects of the inner wall rotation on the unsteady multicellular flow of natural convection in the conductive regime (Ra = 8000). We consider a tall air-filled vertical annulus between differentially heated concentric cylinders with the inner cylinder allowed to rotate. The unsteady Boussinesq equations are discretized using a finite volume method with a second order time stepping scheme. The natural convection flow is axisymmetric in this regime, whereas it is known that the mixed convection flow becomes 3D over a range of Reynolds number. We observe the transition in a range of Reynolds number close to the critical Reynolds number of the Taylor-Couette flow. The rotation has a weak influence on the axisymmetric time-periodic natural convection flow before the transition, whereas the flow becomes 3D and chaotic after.